This invention relates to an improved process for the preparation of meta-substituted diaryl ethers especially m-phenoxytoluenes. The compounds prepared are known in the art to be useful in a variety of commercial applications, as heat transfer agents, and as intermediates in the preparation of synthetic pyrethroid pesticides.
Various processes for the preparation of diaryl ethers, have been reported in the literature. In general, the processes of the prior art can be described in terms of two classes of process. In one class of prior art processes, the product obtained from two aryl reactants is primarily a single diaryl-ether compound produced by nucleophilic substitution of a phenol (phenolate) compound for a halogen on the nucleus of the second aryl reactant, at the site of the halogen atom. In the second class of processes, the diarylether product is an isomeric mixture, at least partially derived from a single starting material. The process typically involves the formation of a nucleophile by hydrolysis of an aryl reactant followed by reaction of the nucleophile with a second molecule of the aryl reactant.
U.S. Pat. No. 1,099,761 discloses the preparation of aryl ethers by the reaction of an aromatic halogenated hydrocarbon with a dry alkali phenolate dissolved in phenol with heat and pressure. In this manner the patent specifically teaches the preparation of m-cresylphenyl ether (M-phenoxytoluene) by reaction of chlorobenzene with dry m-potassium cresylate dissolved in m-cresol.
U.S. Pat. No. 3,032,594 discloses the preparation of dinitrodiphenyl ether by reaction of an alkali metal salt of nitrophenol with chloronitrobenzene in dimethyl sulfoxide solvent.
U.S. Pat. No. 3,634,519 teaches a process for the production of diarylethers by reaction of a p-halo-nitro aromatic compound with an aqueous solution of an alkali metal hydroxide in a polar organic solvent such as dimethyl sulfoxide.
U.S. Pat. No. 3,755,467 discloses the preparation of halogen-containing aryl ethers by a process which comprises the reaction of an alkali metal phenolate with a trihalobenzene in the presence of a strongly polar organic solvent, such as dimethyl sulfoxide, at an elevated temperature.
Japanese Pat. No. 74/62432 discloses the preparation of m-phenoxytoluene by reaction of an alkali metal salt of m-cresol with chlorobenzene in the presence of an organic base and a catalytic amount of copper or copper compound.
U.S. Pat. No. 4,092,364 discloses a process for the preparation of diphenyl oxide by caustic hydrolysis of chlorobenzene and recycling of by-product phenol to maximize the yield of diphenyl oxide as well as the yield of phenylphenol and biphenylphenyl ether by-products.
In British Pat. No. 1,191,409 it is disclosed that ditolyl ether mixtures may be prepared in an industrial scale by the reaction of chloroetoluenes alone or mixtures of chlorotoluenes and cresols in the presence of an aqueous sodium hydroxide solution at temperatures of 250.degree. to 400.degree. C. and under pressures of 100 to 300 atmospheres. The process disclosed is non-selective and thus produces a mixture of ditolyl ethers.
Although various processes for the preparation of diaryl ethers are known in the prior art, it will be appreciated that still further improvements are desirable, such as improvements in product selectivity and purity, the use of more economical reactant, and less stringent reaction conditions, such as lower reaction pressures.
It is an object of the present invention to provide an improved process for the preparation of meta-substituted diaryl ethers. It is a further object to provide an improved method for the preparation of meta-substituted diaryl ethers wherein the aryl groups differ from each other. It is a still further object to provide an improved method for the preparation of meta-substituted diaryl-ethers wherein at least one of the starting materials is a readily available ortho-haloaryl isomer. It is a more specific object to provide an improved method for the preparation of m-phenoxytoluene. It is a still further object to provide such a process for the preparation of m-phenoxytoluene from readily available and economically attractive reactants, such as phenol and ortho-chlorotoluene.